Sound is an undesired result of many desirable functions. The control of undesired sound is important in any number of devices. Without some control of sound emitted, for example, by modern devices, many modern environments would be largely intolerable to people. Be it the household, the office, the inside of a vehicle, a manufacturing plant, everyday devices produce noise that must be controlled.
One aspect of noise reduction is to make devices and systems that inherently produce less noise. For example, in computers a solid state memory produces little to no noise when compared to a disk drive. Similarly, an LCD display produces little to no noise when compared to a CRT.
In many instances, however, noise creating features cannot be eliminated. Examples of noise producing devices include motors and fans, both of which are often necessary to provide desirable operations. Similarly, power supplies, transformers, and other device components produce noise. Circulating liquids, in fluid or gas form, also create noise. Component heating and cooling create noise, such as noise emitted when plastic and metal parts cool from high temperature. Accordingly, canceling noise after it is created is often important.
Passive noise cancellation includes sound absorbing materials. These are highly effective. However, for many reasons, there is an increased interest in active noise cancellation. An active noise cancellation system may be, in some instances, more efficient and less bulky than passive noise cancellation. There remains a need for an improved active noise cancellation.
Many systems that require noise control exhibit two types of disturbances: periodic and non-periodic. Recently, work in the area of repetitive control has produced good results in the rejection of periodic disturbances. Repetitive controllers can be viewed as an extension of the internal model principle. An internal model, often called a memory loop, is placed in the feedback loop in order to cancel the repetitive disturbance. Since the standard memory loop is marginally unstable, it is impractical to implement without modification. Typically, two filters are used to modify the memory loop. One filter is used to create a stable model, and one filter is used to eliminate high frequency components. This method results in a high order internal model that is designed on a trial and error basis. Additionally, non-periodic effects are often left out of the analysis, and the resulting controller can over amplify these components.
The invention is directed to methods and systems to address these needs.